1. Field of the Invention
The present invention is directed to a method for coating nuclear fuel pellets and more particularly to a method for coating nuclear fuel pellets utilizing a metal oxide formed in accordance with the sol-gel process.
2. Description of the Prior Art
At present, nuclear fuel pellets, generally composed of uranium oxide (UO.sub.2), are coated with zirconium diboride (ZrB.sub.2) to produce integral nuclear fuel burnable absorbers. The zirconium diboride coating is applied to the fuel pellets in order to absorb the neutrons released during the burning of the fuel, and thereby provide control of core excess reactivity in the beginning of the fuel cycle.
In operation, fuel rods are produced by packing the coated nuclear fuel pellets in long cylindrical containers. These cylindrical fuel rods are then inserted into the nuclear reactor core to supply the fuel for the nuclear reactor.
When the nuclear fuel pellets coated with zirconium diboride are irradiated in the reactor, the boron in the coating is decomposed into lithium and helium. As the boron decomposes, helium is generated and released into the plenum in the fuel rod. Thus, use of integral fuel burnable absorber rods causes additional rod internal pressure due to the generation of helium during irradiation, as compared to standard fuel rods. As nuclear plants seek higher burnups, longer fuel cycles, and greater fuel enrichment, there will be a need for greater reactivity control during the beginning of the fuel cycle. If this were to be achieved with increased zirconium diboride loading, there will be a substantial increase in rod internal pressure due to helium generated from B-10 transmutation during irradiation, which in turn will limit the life of the integral fuel burnable absorber rods. In order to achieve increased reactivity control while reducing the end of life rod internal pressure, an alternative absorber coating is needed, which will reduce helium generation.
A zirconium diboride coating is usually applied to the uranium oxide fuel pellet by means of a sputtering process. This necessitates vacuum deposition which is time-consuming and costly. Zirconium diboride target preparation for the coating is expensive. In addition, the equipment necessary for carrying out the sputtering process and pumping necessary to produce the vacuum create expensive capital and operating costs. Moreover, the thickness of a coating applied by means of a sputtering process cannot be consistently maintained nor can a sputtering process be used to apply a coating on an irregular-shaped surface. Consequently, there is a need for an improved process for applying a coating on a nuclear fuel pellet.